Analysing the Impact of Roof Obstructions on Daylight Levels in Atrium Buildings: Physical Scale Model Measurements under Real Sky Conditions

Transcription

1 2011 2nd International Conference on Environmental Science and Technology IPCBEE vol.6 (2011) (2011) IACSIT Press, Singapore Analysing the Impact of Roof Obstructions on Daylight Levels in Atrium Buildings: Physical Scale Model Measurements under Real Sky Conditions Julitta Yunus 1, Sabarinah Sh Ahmad 1, 2 and Azni Zain-Ahmed 3 1 Faculty of Architecture, Planning & Surveying, 2 Research Management Institute, 3 Centre of Research in Innovation and Sustainable Energy, Institute of Science, Universiti Teknologi MARA, Shah Alam, Malaysia 1 julitta@salam.uitm.edu.my, 2 sabar643@salam.uitm.edu.my, 3 azniz132@salam.uitm.edu.my Abstract This paper describes the effects of atrium roof forms and internal structural obstructions (roof truss) upon the daylight levels in atrium buildings under tropical real sky conditions (intermediate sky). Daylight measurements were made inside one atrium model with four different types of structured roof forms (flat, pitched, pyramidal-gridded and south facing saw-tooth). Results showed intermediate skies revealed a generally linear relationship and had a good degree of correlation with the overall reduction of daylight levels. Internal obstructions tend to reduce about 55% of light transmittance with similar pattern of losses for all roof profiles. It was observed that the internal obstructions of roof created relatively constant attenuation of the daylight compared with the clear unobstructed roof. The structured flat roof had the least impact on roof transmittance, and generally all four roof forms had similar patterns of light losses. The measurements also confirmed that as the sky brightened, the daylight availability became more significant as the inclined glazing and internal obstructions tend to reflect or splay sunlight away or into the atrium. The analysis showed significant differences between the effects of roof forms and internal roof obstructions (roof truss) on daylight levels that will support the authors future project of using innovative glazing panels as roof glazing material. Keywords-Daylight, Atrium, Roof Transmittance, Scale Model, Real Sky Conditions I. INTRODUCTION Daylighting represents one an aesthetic quality of atria and is an essential component of visually appealing and energy-efficient atria buildings. Key issues for daylighting are daylight availability, distribution and utilization. Therefore, the atrium roof form and structure, its orientation, the reflectivity of the atrium surfaces and the glazed area within the space, will influence the lighting energy consumption. To achieve optimum daylight performance, it is necessary to have a good understanding of how daylight is transmitted into a space, where a roof form and structure are important design elements of the atrium building. As a consequence, such a careful design of the roof fenestration system will then influence and improve daylight availability within the space. The amount of daylight entering an atrium through external roof envelopes depends on the form and height of the atrium, transmission of the glazing, reflectance of the atrium walls and the transmittance of the atrium roof. This paper will focus on roof structure systems since this is perhaps the least understood area of atrium design [1-3]. This initial investigation revealed real-time evidence on the measured effects of roof obstructions upon daylight levels in atria under real sky conditions using physical scale models. II. BACKGROUND An atrium s characteristic is the roof: a careful design of the roof fenestration systems limits glare, mitigates passive solar heating effects and supplies adequate daylighting and minimizes sunlight. A common feature of most studies was that the atrium roof was left as an open, unobstructed void, which is obviously very different from the conditions existing in real atria. Therefore, this is an area not studied before [4]. However, there were significant series of investigations on daylight performances for atria buildings that incorporated roof forms and internal obstructions as their atria roof transmittance variables [1, 5-7]. Although these works described an experimental study on the effect of atria roof forms under real sky conditions, only one type of fenestration system was considered and study was focused on temperate regions. Indeed, these studies recommended further analysis on several variations of roof structures and roof glazing and their impact on the distribution of daylight. Physical scale model experiment have been used as the method of investigation since it assesses luminance levels under real sky conditions [8]. This reliable method provides incisively quantitative and qualitative results to support decision-making process on daylight performance for buildings. The justification for specifying tropical sky is that the Malaysian Sky was discovered to be of the intermediate type [9, 10]. This type is very different from the established overcast or clear sky conditions of Europe and North America. Considerable research has been conducted for the temperate climate region under overcast sky conditions, however, the environmental objectives of an atrium in a tropical and temperate region are not the same and today s passive daylighting system requires different approach than the conventional [11, 12]. This paper investigates the effects of roof forms and internal structural obstructions on daylight levels in atria. The findings will be used to facilitate further investigations V1-279

2 in which targets innovative use of atrium for better daylight performance in office buildings under tropical sky conditions. III. METHODOLOGY A. The Physical Scale Model under Real Sky Conditions A 1:50 scaled model was used to investigate the daylight distribution and illuminance level of an atrium building with different roof fenestration designs (Fig. 1). The model was scaled to 1:50 as it is the most appropriate size [8] to simulate a square four-sided top-lit atrium. Its full-scale dimensions were 400m 2, with four floors of 3.0m height each and measured for sun and skylight distributions under real sky conditions. The model was made of 12mm thick MDF board, with internal size of 400mm x 400mm x 240mm, with a Plan Aspect Ratio (PAR) of 2, a Section Aspect Ratio (SAR) of 2 and Well Index (WI) of 1.0. The internal sides of the model were painted matt black to minimize internal surface reflections (about 2%) and to focus on roof transmittance. The external walls were painted with white gloss to maximize external surface reflectance (about 85%). The model was tested under real sky conditions in Shah Alam, Selangor, Malaysia. Since the model was solely top-lit, it was placed on 1.5m high platform, at the roof-top of a 4- storey building, to avoid obstruction from surrounding area (Fig. 2). Figure 1. Four types of atrium roofs with structural obstructions used in the investigation. Figure 2. Physical model of atrium on the platform, placed at the rooftop. Four types of roof forms were used in the investigation: a) structured flat roof, b) structured pyramidal-gridded roof, (roof height 60mm, pitch angle 18 0 ), c) structured northsouth facing saw-tooth Roof and d) structured north-east sloping glazed pitched roof (roof height 165mm, pitch angle 18 0 ). All roofs were constructed with 2mm thick clear glazing and white-painted wooden struts that represent roof trusses. The selection of roof forms examined were based on typical built atrium for office building in Malaysia [13]. This study was inspired by these projects [1, 6], where their results showed under overcast sky conditions, the structured flat, pitched and mono-pitched roof, performed similarly and likely to have a linear relationship with good degree of correlation. B. The Equipment Set-up and Illuminance Measurement The physical scale model was equipped with six newly calibrated illuminance sensors EKO Luxmeter (ML020O) for outdoor illuminance monitoring under real sky conditions. Table 1 shows the details of the experiment. Horizontal measurements were made on the atria well floor (centre, corner and central edge positions). Vertical measurements were taken from three sensors positioned on the central line of walls with heights of 60mm, 120mm and 180mm above the floor level (Fig. 3). All measurements were kept constant by drilling 22mm diameter holes for six sensors, and the top surfaces of the sensors were pushed through the hole so that the sensors surface were flushed with the wall. Thus, 16 illuminance measurements were made in each model for each roof profile per half hour. TABLE I. TABLE 1 DETAILS OF EXPERIMENTS UNDER REAL SKY CONDITIONS Placement On an flat platform, raised Roof Top, Block C, FSPU (Fig. 2) 1.5m Sky type Real Sky Type of Detailed quality study model Scale 1:50 Location Address Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia Latitude 3 o Longitude 101 o Elevation 32m Climate Average Temperature (min) 23.4 o Average Temperature (max) Annual Solar Radiation Annual Cloud Cover Annual Average Sunshine hours 32.5 o MJm Oktas 12 hours All internal illuminance measurements were set for the various combinations of roof type, sky type, four-ordinance orientation and position of sensors in each model. The illuminance sensors were attached to independent data logger (datataker DT80) for all internal measurements. Three sensors were positioned horizontally and another three vertically (Fig. 3). The sensors were connected to six individual channels that gave outputs corresponding to a range of illuminances for each individual luxmeter. Some of the channels had to be modified to measure to over 130,000 lux. A data logger, connected to a portable computer, recorded the illumination level of each sensor using a computer program called DeLogger5. Extech Illuminance V1-280

3 Meter (HD450) with built-in memory was used for external illuminance measurements that ran concurrently with the internal illuminance measurements. Illumination levels were logged every minute, repeated for four-ordinance orientation for each type of roof, between 8.00am and 6.00pm under real sky conditions. Figure 4. Illuminances for unobstructed and structured flat roof under real sky conditions. Figure 3. Atrium dimensions and illuminance sensors vertical and horizontal positions IV. RESULTS Results from the measurements were quantified based on distribution of light within the atrium interior including average illumination, variance within daylight hours and minimum and maximum illumination. Then these datasets were analyzed three ways: Overall reductions in daylight levels created by the roofs were derived by plotting internal against external illuminance for each roof type for a number of sensor positions, comparing each plot with the simultaneous plot from the clear unobstructed model. The plotted value described the Daylight Factor (DF) distribution on the horizontal and vertical surfaces. The effects of roof structure upon the daylight reduction at atrium floor level by plotting the curve of illuminance levels over the daylight hours for each atrium roof configuration. The pattern of daylight transmittance for atrium floor level illuminance over the daylight hours for each atrium roof configuration were derived by plotting percentage of light losses due to different types of roof configurations with the daylight hours. A. Overall reductions in daylight levels created by the roof structures Overall reductions in daylight levels created by the roofs was derived by plotting internal against external illuminance for each roof type for a number of internal sensor positions and comparing each plot with the simultaneous plot for the clear unobstructed model. Intermediate sky was considered in this study as it represents the most common sky condition for the geographic location of this study. The plotted value showed Daylight Factor (DF) distribution on the horizontal and vertical surfaces of the atrium model. Fig.3, 4, 5 and 6 show the gradient lines of measured daylight illuminance levels at the atrium floor level and concurrent external horizontal illuminance (under intermediate sky conditions) for each roof type. Figure 5. lluminances for unobstructed and structured pitched roof under real sky conditions. Figure 6. lluminances for unobstructed and structured pyramidal roof under intermediate sky conditions. Figure 7. lluminances for unobstructed and structured saw-tooth roof under real sky conditions. The results plotted showed a generally linear relationship and good degree of correlation. The gradient of each linear regression represented the sky component of the daylight factor for each particular roof configuration. For example, Fig. 3 shows the value of visible sky obtained from the sensors in the atria with structured flat roof was 33% (R 2 =0.937). From the overall analysis of four different types of roof forms, it was evident that under intermediate sky V1-281

4 conditions, there were losses on daylight availability caused by the internal roof trusses and glazing bars. As a result, the structured flat roof had the least impact on daylight levels in the atrium well with overall contribution of 33%. The inclination of glazing panels and structural obstructions contributed between 4% and 7% lesser visible sky component for the structured Pitched and Saw-tooth roofs. B. The effects of roof structure upon the daylight reduction at atrium floor level This section describes the pattern of light losses due to effects of roof structure upon daylight levels in the model atrium. However, this condition does not signify the distributions of light in the atrium as they correspond with other parameters. Based on the measurement taken, the proportion of visible sky seen from the sensor position was only 3% for the saw-tooth roof. The area indicated as visible sky did not provide the corresponding illuminance levels compared to if there was an open top well, due to reflectance and absorption by the glass. Likewise, the area designated as obstructed from the light contributed to the illuminance measured at the sensor due to inter-reflections from the surfaces. In this case light gained from perceived obstruction (i.e. the structure and the large opaque wall obstruction) outweighed the losses due to attenuation on opaque surfaces. Until the exact relationship between the glazing transmittance value and structural reflectance value can be assessed, it is impossible to implicitly state the relationship between the proportion of visible sky and daylight levels at a given point. Reductions of daylight availability caused by roof structural elements were evident. The pattern of light loss was similar for all four types of roof. The analysis showed that the roof structures created a relatively constant attenuation of daylight penetration on the floor of the atrium compared with the type of roof and internal obstructions. The results showed that each roof type measured under cloudy sky conditions experienced about 55% daylight reduction at floor level, which agreed with [14], where they proved that structures of glazing bars reduced daylight between 20% and 80% where typically 50% were lost in passing through the roof. The decreasing pattern is similar for four types of structured roof model, with light reduction between 37% and 46%. The Structured Flat Roof had the least impact on daylight transmittance with only 37% daylight reduction compared to north-south Structured Pitched Roof and Structured Pyramidal-Gridded Roof, both 45% and 46% respectively. While south-facing structured Saw-tooth Roof reduced more than 90% of daylight. The measurements were not taken simultaneously though, so it cannot be ruled out that the light level was lower due to changes in sky conditions. Fig. 7, 8, 9 and 10 showed the variations of daylight illuminance level across the centre of the atrium floor for the structured Flat, Pitched, Pyramidal-Gridded and Saw-tooth Roof compared to clear unobstructed atrium. The pattern of losses was similar for the four types of roof under real sky conditions measurements. The structured flat roof had the lowest rate of light losses due to its relatively higher amount visible sky component compared to structured pitched roof and structured pyramidal-gridded roof. The latter twostructured model atrium well appeared less lit as some of the light was deflected out of the glazing bars for high elevation angles. This noticeable evidence led to further investigations to modify the roof configurations and glazing material. Thus, future investigation derived from this investigation will include innovative glazing material to improve the daylights transmittance in the atrium [2, 15, 16]. Figure 8. Daylight levels at the centre of floor level for Structured Flat Roof and Unobstructed atrium Figure 9. Daylight levels at the centre of floor level for Structured Pitched Roof and Unobstructed atrium Figure 10. Daylight levels at the centre of floor level for Structured Pyramidal-Gridded Roof and Unobstructed atrium Figure 11. Daylight levels at the centre of floor level for Structured Sawtooth Roof and Unobstructed atrium V1-282

5 Further measurements on illuminance level were taken for north, west, south and east orientations with the same parameters for roof forms and internal obstructions. The results show that the average illumination level in the atrium have the same pattern for all roof types which were higher at the floor level compared to the vertical surfaces for low, middle and high-level wall. Table 2 shows the general pattern on average vertical illuminance data for low, middle and high-level wall. Generally, the horizontal surfaces received higher average illuminance levels compared to vertical surfaces. It was revealed that under intermediate sky conditions, south facing surfaces received higher illuminance levels for three types structured roof; flat, pitched and pyramidal-gridded, while structured saw-tooth roof with south facing surface as the highest illuminance levels. As shown in Table 2, daylight level reaching the lowest level is almost three quarter of daylight on the high level. TABLE II. AVERAGE ILLUMINANCE LEVEL MEASURED AT HORIZONTAL FLOOR LEVEL AND VERTICAL WALL FOR FOUR TYPES OF ROOF COMPARED TO UNOBSTRUCTED ATRIUM Unobstructed Atrium Structured Flat Roof Structured Pitched Roof Structured Pyramidal Gridded Roof Structured Sawtooth Roof Average Illuminance Level on atrium s vertical wall Low Middle High North 5, , , West 7, , , South 9, , , East 7, , , North 3, , , West 4, , , South 6, , , East 4, , , North 2, , , West 3, , , South 4, , , East 3, , , North 3, , , West 4, , , South 5, , , East 4, , , North 1, , , West , South East , For higher angles and complicated roof profiles, daylight levels on west and east facing interior decreased by more than half compared with unobstructed atrium, and west facing interior received more lights at all floor levels. Exception to structured pyramidal-gridded where east-facing interior received more daylight for low and high-level atrium wall. It was observed that the obstructed pitched and pyramidal-gridded roof received the least daylight for northfacing interior surfaces. The reductions in daylight levels under these roofs measured between 16% and 28%. Additionally, the reductions of daylights levels for low and mid-level vertical walls were not significant for all type of roof, particularly for structured pitched and structured pyramidal-gridded roofs. While for high-level wall locations the level of daylight were significantly higher than the middle and low levels as the daylights entering the interior space were predominantly direct light and this is obvious for structured flat roof. It is interesting to note that for structured pyramidal-gridded and structured flat roof, the mid-level received less light than low-level wall. This phenomenon showed that the changing roof pitch and form showed different light patterns entering the atrium well. Significantly, the result showed a reversed condition for similar procedure of measurement in temperate climate [6], under overcast and partly cloudy sky conditions, for obstructed A-frame roof, the north facing surfaces received higher illuminance levels. C. The pattern of daylight transmittance for atrium floor level illuminance throughout the day The pattern of daylight losses was similar for the three types of roof under intermediate sky conditions. Fig. 11 shows the percentages of the illuminance losses for the four types of atrium roofs. It can be concluded that under intermediate sky conditions, the internal obstructions of structured flat roof has the least impact on the light losses, followed by structured pitched roof and structured pyramidal-gridded roof. On the other hand, as the sky conditions brightened during midday, the effects of roof shape and internal obstruction on daylight levels became more significant. Between 1100 and 1500, the percentages of light losses constantly decreased for the three structured roof; flat, pitched and pyramidal-gridded. This was due to internal obstruction and inclination of glazed roofs reflecting the bright sunlight away or into the atrium. The percentages of daylight reductions dropped from up to 84% at 0900 to 69% at 1300 and then went up again after However, the level of daylight availability in the atrium of structured sawtooth roof were low compared to the other three roofs as the percentages of illuminance reductions were very high throughout the day. Further analysis showed that, under cloudy sky conditions between 0800 and 1100, the percentage of illuminance reduction levels fluctuated for structured flat, pitched and pyramidal-gridded roofs. Conversely, under overcast condition towards the end of the day, the illuminance levels dropped as the lights that entered the atrium were predominantly diffused lights, thus internal roof obstructions gave the tremendous impact on daylight availability. Figure 12. The percentages of illuminance reduction throughout the day for each atrium roof configuration under intermediate sky conditions. V1-283

6 V. CONCLUSIONS This paper describes an experimental study on the effects of atrium roof form, internal structural obstructions upon the daylight levels in atrium buildings under real tropical sky conditions (intermediate skies). Daylight measurements were made in a 1:50 scale model of an atrium building with four types of structured roofs (flat, pitched, pyramidal-gridded and south facing saw-tooth). The experiments evaluated daylight penetration in the atrium, while the adoption of varied roof forms and internal structural obstructions for the atrium allowed better insight on their effects upon daylight availability horizontally and vertically. The measured data were analyzed and quantified three ways: i) overall reduction in daylight levels created by the roofs, ii) the effects of roof structures upon the daylight distribution in a model atrium building, iii) the percentage of light losses due to different types of roof configurations throughout the day. The intermediate sky conditions showed a generally linear relationship and good degree of correlation on the overall reduction of daylight levels in the atria. Results showed that structured roof forms applied to the atrium model reduced daylight levels in the atrium well. Internal obstructions reduced light transmittance by 55% with similar pattern of losses for all roofs. Internal roof obstructions created a relatively constant attenuation of daylight compared with the clear unobstructed roof for the four structured. It was evident that structured flat roof had the least impact on roof transmittance, and generally all four roofs forms had similar pattern of light losses. However, the inclined roof produced different pattern of daylight reduction levels especially on the vertical surfaces of the atrium well. It was established that for higher angles and complicated roof profiles, west and east facing surfaces, the daylight level decreased more than half compared with unobstructed atrium, and west facing atrium received more lights at all floor levels. The results also confirmed that as the sky brightened, the daylight availability become more significant as the inclined glazing and internal obstruction reflected sunlight/daylight away or into the atrium. During the brightest time of the day, the daylight reduction levels were constantly decreasing for the three structured roof; flat, pitched and pyramidal-gridded. However, the structured saw-tooth roof needed further roof modifications as it had dissimilar light transmittance values. This study showed the results of an initial investigation on the effects of the atrium roof form and internal obstructions on daylight levels in the atrium. Thus, only one parameter, which is the roof structural obstruction, was considered in the investigation. The analysis also did not consider the effects of clear sky conditions, as they were very difficult to find and data measured showed the intermediate sky was found to be a norm in this tropical location. Finally, was essential to obtain empirical daylight transmittance data to assess the validity of proposed atrium roof forms and glazing profiles, before any future proposals to adopt innovative glazing materials for the atrium could be considered. ACKNOWLEDGMENTS This study was funded by the Science Fund from the Ministry of Science, Technology and Innovation, Malaysia. REFERENCES [1] S. Sharples and A. D. Shea, "Roof Obstructions and Daylight Levels in Atria: a Model Study Under Real Skies," Lighting Research and Technology, vol. 31, pp , [2] S. Sharples and D. Lash, "Daylight in Atrium Buildings: A Critical Review," Architectural Science Review, vol. 50.4, pp , [3] P. J. 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